There has been known a need in the industry for a nut that is captive within a sheet yet freely rotatable so it can be press-inserted without the need for additional structures or assembly. Examples in the prior art are disclosed in U.S. Pat. No. 3,374,382 issued to Bentrim and U.S. Pat. No. 5,842,894 issued to Mehlberg. These documents disclose a rotatable captive nut that is mounted flush with the backside of the sheet and which relies upon the deformation of the sheet for captivation by the nut as it is pressed in. Inherently, these nuts require the attachment sheet to be of relatively soft material so that features on the nut will displace sheet material into an undercut groove which captivates the nut to the sheet. In order to meet the needs in the art for a rotatable captive nut for sheets of greater hardness as disclosed in U.S. Pat. No. 6,932,044 issued to Fenech, additional structures such as a washer on the backside of the sheet are employed. However, this negates the desired flush mounting with the backside of the sheet.
Captive nuts which are pressed into sheets of relatively hard material that provide flush but rigid attachment require a two-step pressing process in which the sheet is first deformed into a generally dome-shape or frustoconical-shape surrounding the sheet hole. In a separate second step, a tubular shank extending from the backside of the nut is inserted through the hole and flared over the sheet. The flared-over-section of the end of the nut occupies the void in the backside of the sheet formed by the panel deformation of the first step thus providing flush attachment. Examples of such methodology are disclosed in U.S. Pat. No. 5,423,645 issued to Muller et al. However, in these cases, the nut is not rotatable and the double pressing requirement makes the assembly process expensive.
It is therefore an object of the present invention to provide a rotatable nut attachment to a sheet of greater hardness which is characterized in that the nut is either flush or sub-flush with the backside of the sheet. It is a further object of the invention to achieve an insertion process for such a fastener assembly which is a “one-hit” installation using a punch press.